Devices for measuring thickness and refractive index of thin films and bulk materials at room temperature are well known. FIG. 1 depicts such a device 10, of which the “METRICON®” Model 2010 “PRISM COUPLER” is an example.
In such a device, a sample 30 is brought into close contact with the base 12B of a prism 12. Typically, a small air gap 18 will form between the sample 30 and the prism 12. As depicted in FIG. 1, the sample 30 may be a film 32 supported by a substrate 34. The sample 30 may be brought into contact with the base 12B of the prism 12 via a coupling mechanism 14, which may have a coupling head 14A as shown.
Incident light 20 is directed onto an entrance face 12A of the prism 12. The incident light 20 encounters the entrance face 12A at an angle θ and is typically deflected as it enters the prism 12. The deflected light 22 travels through the prism 12 and strikes the base 12B of the prism 12 at an angle of incidence θ1 (which, as shown in FIG. 1, may be measured relative to the normal axis z, i.e., the axis perpendicular to the base 12B of the prism 12). The reflected light 24 travels through the prism 12 and exits the exit face 12C of the prism 12. The reflected light 24 is typically deflected as it exits the prism 12. A detector 16, which may be a single photodetector element or an array of photodetector elements, such as a CCD, for example, is situated proximate the exit face 12C of the prism 12. The detector 16 receives the deflected reflected light 26.
At most angles of incidence θ1, the deflected light 22 will be nearly totally reflected at the prism base 12B. However, at certain angles of incidence θ1 on the base 12B of the prism 12, light 36 will couple efficiently into the film 32 and propagate down the film 32 in an optical propagation or substrate mode. An optical propagation mode occurs when the film 32 is on a substrate or underlying film of lower refractive index. A substrate mode occurs when the film 32 is on a substrate of higher index. These angles, which are known as “mode angles,” are related to the thickness and index of the film 32. At the mode angles, where light 36 is coupled into the film 32, the light 26 reflected to the detector 16 on the exit side 12C of the prism 12 is reduced (see FIG. 2). This provides a mechanism for detecting the optical or substrate propagation mode. Detection of two modes allows calculation of film thickness and index (two equations in two unknowns). If the film possesses more than two modes, each pair of modes allows an additional independent estimate of thickness and index to be made. Any of a number of well-known mechanisms may be provided for varying the incident angle of the light.
There are many applications in which the sample is to be used in an environment where the temperature may differ, sometimes significantly, from room temperature. For example, the sample might be designed for outdoor use (e.g., automotive, electrical supply, etc.). For such an application, it may be desirable to determine thickness and index at temperatures below room temperatures. In other applications, such as packaged electronic devices, for example, the sample might be designed to operate in a heated environment. For such an application, it may be desirable to determine thickness and index at temperatures up to 200° C. or more.
In the prism coupling technique, measurement of index at elevated temperature is complicated by the fact that the sample being measured is in intimate contact with the prism on the front side and the coupling mechanism on the back. Temperature gradients across the prism/sample/coupling mechanism interface create uncertainty in the temperature of the sample. In addition, accurate knowledge of the prism index is required for accurate sample measurements because the refractive index of the prism also changes with temperature. Any gradient in the prism temperature causes uncertainty in the prism index at the point of coupling into the sample, which affects measurement accuracy.
It would be desirable, therefore, if apparatus and methods were provided for measuring thickness and refractive index of thin films and bulk materials at temperatures other than room temperature. Such apparatus and methods would be particularly useful if they were operable to determine thickness and/or index of refraction of such samples at various temperatures.